Dense gas techniques provide a suite of clean technology options for the processing of pharmaceuticals. Monodisperse, micron-sized particles can be produced at mild operating temperatures and with negligible solvent residue. In this study, protein was precipitated from organic solutions using dense carbon dioxide as antisolvent. The gas antisolvent precipitation process (GAS) was used to produce biologically active lysozyme, insulin, and myoglobin powders. The effects of operating temperature, solute concentration and the rate of antisolvent addition on the morphology, size, activity and residual solvent concentration of lysozyme and insulin precipitates have been examined. The powders produced consisted of uniformly sized non-aggregated spherical particles. Precipitate size was controlled between 0.05 mu m and 2.0 mu m by changes to the solvent and antisolvent compositions. In general the concentration of residual organic solvent was found to be dependent on the mass of antisolvent used during the washing cycle. Residual concentrations as low as 300 ppm were easily achievable in a single step.